A search and rescue operation involving a diving team, a helicopter, one coastguard ship and seven boats is under way to search for the missing migrants.

Fishermen are also joining in on the search. Many of those who have been pulled from the water are children.

"They had life jackets. But there were bodies everywhere. Babies, children... We pulled out 15 to 20 bodies," fisherman Kadir Sert told Hurriyet news.

Ali Saruhan, one of the rescuers involved in the search, added: "The wind is making our task very difficult. The boat is a very small one, it's in fact just a small boat. But they were carrying 40 people in it. We are seeing bodies of children floating in the sea."

It is believed the ship carrying around 50 migrants was heading to the Romanian port of Constanta when it sank around three miles (5km) off the coast. It is said overcrowding and bad weather caused the boat the sink.

The Afghan migrants on board the boat paid several thousand euros to enter the EU after setting off from the Istanbul suburb of Bakirkoy, reports AFP.

More than 3,000 migrants died trying to cross the Mediterranean and Aegean so far this year, according to the International Organization for Migration, but few have been reported to make it across the Bosphorus.

In September, a boat carrying 82 migrants, mostly Syrians and Afghans, was rescued by the Turkish Coast Guard off the northern coast of Istanbul. The migrants were also believed to be heading towards Romania.